Motility induced changes in viscosity of suspensions of swimming microbes in extensional flows

TL;DR

This study investigates how motility affects the viscosity of microbial suspensions in extensional flows, revealing that motility can either increase or decrease viscosity depending on the microorganism type, supported by experiments and extended modeling.

Contribution

The paper provides a quantitative comparison of theory and experiment for microbial suspension rheology, extending a recent model to higher concentrations and highlighting shape effects.

Findings

Motility increases viscosity in algal pullers.

Motility decreases viscosity in bacterial and sperm pushers.

Shape details significantly influence rheological behavior.

Abstract

Suspensions of motile cells are model systems for understanding the unique mechanical properties of living materials which often consist of ensembles of self-propelled particles. We present here a quantitative comparison of theory against experiment for the rheology of such suspensions. The influence of motility on viscosities of cell suspensions is studied using a novel acoustically-driven microfluidic capillary-breakup extensional rheometer. Motility increases the extensional viscosity of suspensions of algal pullers, but decreases it in the case of bacterial or sperm pushers. A recent model [Saintillan, Phys. Rev. E, 2010, 81:56307] for dilute active suspensions is extended to obtain predictions for higher concentrations, after independently obtaining parameters such as swimming speeds and diffusivities. We show that details of body and flagellar shape can significantly determine macroscale rheological behaviour.